The project is currently directed toward characterizatton and purification of the p21 src protein coded for by the rat-derived RNA sarcoma viruses including Harvey and Kirsten strains of murine sarcoma viruses and a rat sarcoma virus of pure rat origin. The fact that p21 is indeed the transforming src protein of these viruses has been established by study of this protein (1) in a Ki-MSV mutant (ts371) temperature-sensitive for the maintenance of transformation and (2) in cells transformed by molecularly cloned Ha-MSV DNA. An enzymatic activity catalyzing a GTP-specific autophosphorylation of the p21 src protein has been found in the purified p21 protein. An endogenous p21 homologous to the viral p21 src has been found in many normal vertebrate cells including human. The p21 src has been localized to the inner surface of the plasma membrane of virus-transformed cells suggesting such a cellular location the major site of action of this transforming protein.